1. Field of Invention
The present invention relates to a signal processing circuit, for example, a quadrature modulator in a mobile communication apparatus etc. and a gain controlled type amplifier (GCA) connected to the same for gain control. The present invention further relates to a communication apparatus using such a signal processing circuit.
2. Description of the Related Art
Japanese Patent Publication (A) No. 11-136051 and Japanese Patent Publication (A) No. 8-223233 disclose communication apparatuses having transmission circuits modulating and amplifying the base band (BB) signal of a code division multiple access (CDMA) type mobile phone and emitting it from an antenna. The communication apparatus 200 illustrated in FIG. 7 has a modulation circuit 201 having an I- and Q-quadrature modulator 202 including a mixer and converting frequency and a plurality of gain controlled type amplifiers 203A to 203C, a surface acoustic wave (SAW) filter 205, a power amplifier 206, a duplexer 207, and an antenna (ANT) 208. The communication apparatus 200 converts in frequency a base band (BB) signal comprised of two orthogonal signals with phases in an orthogonal relationship, that is, an in-phase (I) signal and quadrature phase (Q-phase) signal to convert it into a high frequency (RF: radio frequency) signal able to be emitted from the antenna 208, amplifies it to a predetermined level, and emits it from the antenna 208 into the air. The BB terminal to which the base band (BB) signal is supplied is connected to the I- and Q-quadrature modulator (MOD) 202. The I- and Q-quadrature modulator 202 converts the base band signal to a 800 MHz or 2 GHz band RF signal. The output of the I- and Q-quadrature modulator 202 is connected to the GCA circuits (gain control amplifiers) 203A to 203C, and the RF signal output from the I- and Q-quadrature modulator 202 is amplified. For example, with one stage of the GCA circuits, there is a gain of about 30 dB. For the antenna (ANT) 208 to emit a signal, for example, a gain of about 80 dB is necessary, so three stages of GCA circuits 203A to 203C are provided. The amplified output of the GCA circuit 203C is connected to the SAW filter 205, the transmission signal is stripped of unnecessary high frequencies by selection of the frequency at the band pass filter of the SAW filter 205, and only the signal of the desired frequency band is output. The transmission signal output from the SAW filter 205 is amplified in power at the power amplifier 206, supplied through the next stage DUP (duplexer) 207 to the antenna 208, and emitted into the air.
For example, in the case of a CDMA type mobile phone, a range of gain control with respect to the base band signal BB of about 80 dB or more is considered necessary. Further, good control linearity and temperature characteristics are demanded. Therefore, while there are the above three stages of more of gain controlled type amplifiers (GCA circuit), usually an I- and Q-quadrature modulator 202 does not control the gain of the frequency converted signal. That is, the gain control is performed by the plurality of gain control amplifiers 203A to 203C provided after the I- and Q-quadrature modulator 202. The plurality of gain controlled type amplifiers 203A to 203C 3 connected after the I- and Q-quadrature modulator 202 are configured as shown in FIG. 8.
FIG. 8 shows the configuration of one circuit of the plurality of gain controlled type amplifiers 203A to 203C. The gain controlled type amplifier 250 shown in FIG. 8 is comprised as a differential pair type amplification circuit, an emitter of the NPN transistor 251 and an emitter of the NPN transistor 252 are commonly connected, a load Z1C is connected between a collector of the NPN transistor 251 and the supply line Vcc, and a collector of the NPN transistor 252 is connected to the supply line Vcc. A common emitter of the NPN transistors 251 and 252 is supplied with the output signal of the I- and Q-quadrature modulator 202 through for example a voltage-current conversion circuit etc. as a signal current I0. Further, the bases of the two transistors are supplied between them with a control voltage Vc for controlling the gain. The control voltage Vc controls the gain of the gain controlled type amplifier 250 and the signal current I0 and uses the gain to amplify the voltage. The amplified voltage is taken out as the output signal Vo from the collector of the NPN transistor 252.
The circuit operations of the gain controlled type amplifiers 203A to 203C will be explained next. In particular, the operation for showing the magnitude of the temperature fluctuation of the gain controlled type amplifiers 203A to 203C will be explained with reference to the GCA circuit 250 shown in FIG. 8. The output voltage Vo of the collector of the NPN transistor 251 of the GCA circuit 250 shown in FIG. 8 becomes as follows:Vo=Z1×I1  (1)
The following relations stand:Vbe1=Vt×ln(I1/Is)  (2)Vbe2=Vt×ln(I2/Is)  (3)
where, Vt: thermal voltageIo=I1+I2  (4)
From equations (2) and (3), the following equation (5) stands:Vc=Vbe1−Vbe2=Vt×ln(I1/I2)  (5)
From equation (5), equation (6) is obtained:I1=I2×exp(Vc/Vt)  (6)
If entering equation (6) into equation (4) to find I1 and I2, the following equations are obtained:I1=Io/[1+exp(−Vc/Vt)]  (7)I2=Io/[1+exp(Vc/Vt)]  (8)
If entering equation (7) into equation (1), the following equation is obtained:Vo=Z1×Io/[1+exp(−Vc/Vt)]  (9)
If provisionally setting the input voltages Vi=1 and Z1=Io=1 so as to study the gain characteristics of the gain controlled type amplifier 250, the result becomes the gain G shown by equation (10):G=Vo/Vi=1/[1+exp(−Vc/Vt)]  (10)
Regarding equation (10), if changing the ambient temperature and showing the relationship between the control voltage Vc and gain by a graph, the result becomes like FIG. 9. The abscissa shows the control voltage Vc from −0.2V to +0.2V in range in gradients of 0.1V steps, while the ordinate shows the gain in gradients of 10 dB steps from 0 dB to −90 dB in range. Further, the temperature conditions were set to the three conditions of 27° C., −25° C., and 85° C. When changing the control voltage Vc from +0.2V to +0.1V in range, the amount of attenuation is substantially 0 dB regardless of the temperature conditions. Even when the control voltage Vc is 0.1V to 0V in range, the attenuation characteristics are substantially the same, that is, about −6 dB at 0V. When the control voltage Vc becomes 0V or less, temperature dependency of the amount of attenuation appears. When the control voltage Vc is −0.1, the attenuation becomes about −27 dB at an ambient temperature of 85° C., about −34 dB at 27° C., and about −41 dB at −25° C. The difference becomes, at the maximum, 10 dB or more. This shows large fluctuation with respect to temperature. When the control voltage Vc becomes −0.2V, the attenuation becomes about −54 dB at 85° C., about −67 dB at 27° C., and about −82 dB at −25° C. The difference becomes, at the maximum, 25 dB or more. This shows larger fluctuation with respect to temperature. In this way, the GCA circuit 250 shown in FIG. 8 has points which should be improved in control linearity and temperature characteristics.
The plurality of gain controlled type amplifiers (GCAs) 203A to 203C in the modulation circuit 201 explained with reference to FIG. 7, FIG. 8, and FIG. 9 form a three-stage configuration, so the consumed current becomes larger. Further, since a three-stage configuration is employed, the number of elements increases and, in case of formation on an IC, the chip area occupied becomes larger and other problems arise. Further, if configuring the GCA circuit as shown in FIG. 8, there are problems in control linearity and temperature characteristics.